Honda-SOHC

Duff switch? Or muck in the brake line restricting pressure to the switch?
Just been and tried mine and the brake light comes on with little pressure at the lever.
I may have a spare switch kicking around somewhere but would need to have a search for it.

If you only need the one then I would have thought an email to the place in Holland to see what they would charge for carriage would be worth a shot, they only quote 7.50 Euros for the part.

There is one here: https://www.mgparts.nl/product/honda-cb400f-kettinggeleider-23132-333-010-guide-primary/
If you need two then maybe buy this one and use as a pattern to follow orcadian's suggestion.

Wiring harness sounds right. Is it part 1 on this diagram?

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Following on from the above, once you have the Part Number (which you did, as you have the Parts List) then try doing a Google search on the number.
It doesn't always work, but often you get some useful results.
Here's one; not cheap (although the cheapest I have seen) but this part seems to be in short supply:
http://www.ebay.co.uk/itm/Honda-CB400-750-Crankcase-Oil-Passage-Cap-Genuine-15332-300-000-/132067752557

.... Ashford Chroming (CB750 luggage rack) chroming OK but not as good as RS. Mark @ RS prefers the simpler parts as he declined to do the luggage rack

I followed a recommendation and had my rack rechromed at Classics & Chrome in Hinckley, Leics; see earlier post in this thread.
It's OK but there is still some signs of pitting around the joints. They said this was due to being unable to get in to these areas to polish the part properly. Other than that the work seemed good and I would probably try them again if I had more work that needed doing.
Having said that I seem to recall some places would not touch the job because of the problems in preparing well - one place wanted to undo all the welded / brazed joints and dismantle the rack to its component parts so they could polish all the pieces properly (an offer which I predictably declined ....).
It does make me wonder what sort of tools they use to do the prep - surely something smaller and hand-held would be able to better get into the awkward to reach places.

Black wire looks to be switched live, and from the wiring diagram only the front stop light switch appears to be plugged in to a double connector so maybe a spare connection there.
If you are planning a socket near the handlebars what sort are you intending to use? Presumably it needs to be weather proof.